4 Brainstem

Surface Anatomy

of the Brainstem . . . . . . . . . . . . . . 119

Cranial Nerves . . . . . . . . . . . . . . . . 120

Topographical Anatomy

of the Brainstem . . . . . . . . . . . . . . 207

Brainstem Disorders . . . . . . . . . . . 223

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4Brainstem

The brainstem is the most caudally situated and phylogenetically oldest portion of the brain. It is grossly subdivided into the medulla oblongata (usually called simply the medulla), pons, and midbrain (or mesencephalon). The medulla is the rostral continuation of the spinal cord, while the midbrain lies just below the diencephalon; the pons is the middle portion of the brainstem. Ten of the 12 pairs of cranial nerves (CN IIIXII) exit from the brainstem and are primarily responsible for the innervation of the head and neck. CN I (the olfactory nerve) is the initial segment of the olfactory pathway; CN II (the optic nerve) is, in fact, not a peripheral nerve at all, but rather a tract of the central nervous system.

The brainstem contains a large number of fiber pathways, including all of the ascending and descending pathways linking the brain with the periphery. Some of these pathways cross the midline as they pass through the brainstem, and some of them form synapses in it before continuing along their path. The brainstem also contains many nuclei, including the nuclei of cranial nerves III through XII; the red nucleus and substantia nigra of the midbrain, the pontine nuclei, and the olivary nuclei of the medulla, all of which play an important role in motor regulatory circuits; and the nuclei of the quadrigeminal plate of the midbrain, which are important relay stations in the visual and auditory pathways. Furthermore, practically the entire brainstem is permeated by a diffuse network of more or less “densely packed” neurons (the reticular formation), which contains the essential autonomic regulatory centers for many vital bodily functions, including cardiac activity, circulation, and respiration. The reticular formation also sends activating impulses to the cerebral cortex that are necessary for the maintenance of consciousness. Descending pathways from the reticular formation influence the activity of the spinal motor neurons.

Because the brainstem contains so many different nuclei and nerve pathways in such a compact space, even a small lesion within it can produce neurological deficits of several different types occurring simultaneously (as in the various brainstem vascular syndromes). A relatively common brainstem finding is so-called crossed paralysis or alternating hemiplegia, in which cranial nerve deficits ipsilateral to the lesion are seen in combination with paralysis of the contralateral half of the body.

Surface Anatomy of the Brainstem · 117	4

Ingeneral,cranialnervedeficitscanbeclassifiedassupranuclear,i.e.,caused by a lesion in a descending pathway from higher centers, usually the cerebral cortex, which terminates in the corresponding cranial nerve nucleus in the brainstem;nuclear,ifthelesionisinthecranialnervenucleusitself;fascicular, ifthelesioninvolvesnerverootfibersbeforetheirexitfromthebrainstem;or peripheral,ifthelesioninvolvesthecranialnerveproperafteritsexitfromthe brainstem. The type of deficit produced depends on the site of the lesion.

Surface Anatomy of the Brainstem

The three brainstem segments, i.e., the midbrain, pons, and medulla, have clearly defined borders on the ventral surface of the brainstem (Fig. 4.1a).

Medulla

The medulla extends from the site of exit of the roots of the first cervical nerve (C1), at the level of the foramen magnum, to its junction with the pons 2.5 3 cm more rostrally.

Dorsal view. The gracile tubercles are seen on either side of the midline, flanked by the cuneate tubercles (Fig. 4.1b). These small protrusions are produced by the underlying nucleus gracilis and nucleus cuneatus of both sides. These are the relay nuclei in which the posterior column fibers of the spinal cord form synapses onto the second neurons of the afferent pathway, which, in turn, project by way of the medial lemniscus to the thalamus. The rostral border of the medulla is defined by a line drawn through the caudal portion of the middle cerebellar peduncles. The floor of the fourth ventricle, or rhomboid fossa, is bounded laterally by the inferior and superior cerebellar peduncles and divided into rostral and caudal portions by the striae medullares, which contain fibers running from the arcuate nuclei to the cerebellum. The caudal part of the floor contains a number of protrusions (tubercles) produced by the underlying cranial nerve nuclei, including the vagal triangle (or “trigone”; dorsal nucleus of the vagus nerve), the hypoglossal triangle (nucleus of the hypoglossal nerve), and the vestibular area (vestibular and cochlear nuclei), while the rostral part contains the facial tubercle, which is produced by the fibers of the facial nerve as they course around the abducens nucleus. The roof of the fourth ventricle is made up of the superior medullary velum, the cerebellar peduncles, and the cerebellum itself.

4 118 · 4 Brainstem

Crus cerebri

Pons

Inferior olive Pyramid

Pyramidal decussation

Anterolateral sulcus

(a) Brachium of the

superior colliculus

(b) Brachium of the

inferior colliculus

Medial geniculate body (hearing)

Lateral geniculate body (vision)

Crus cerebri

	Pulvinar		Tela choroidea of
Mamillary	Tectal lamina		Tela choroidea of
Mamillary	(quadrigeminal plate)		the third ventricle
body	(quadrigeminal plate)		the third ventricle
body
Optic	Medial		Epiphysis
Optic	geniculate		Superior medullary
tract	body		Superior medullary
	Lateral		velum
	Lateral
	geniculate
	body
	Superior


	cerebellar
	peduncle
	Middle	Floor of
	cerebellar	the fourth
	peduncle	ventricle
	peduncle	ventricle
	Inferior		Facial


	cerebellar		colliculus
	peduncle		Vestibular
	Striae medullares of		area
	Striae medullares of		area
	the fourth ventricle		Tubercle of the
			Tubercle of the
	Hypoglossal		nucleus cuneatus
	triangle		Area postrema
	Vagal triangle		Tubercle of the
	Tuberculum		nucleus gracilis
	cinereum		Obex
			Obex